This invention relates to a rupturable safety sealing gasket for a cell which normally prevents electrolyte from leaking therethrough but which allows a gas abnormally developed in the cell to be released therefrom by rupture of the gasket.
A sealing gasket of this type is known as shown in Japanese Patent Publication No. 37-15358, in which a part of the gasket is made thin enough to allow the gas normally developed in the cell to be diffused therethrough. However, when the internal gas pressure of the cell is abnormally increased, the thin part of the gasket is ruptured to release the gas therethrough and to prevent explosion of the cell.
In spite of such proposal as above, however, since the sealing gasket is practically formed by injection molding of thermoplastic resin, such as polyethylene, polypropylene, or nylon, the thickness of the thin part of the gasket could not be made less than about 0.25 mm, because the fluidity of the resin is obstructed by a thickness of less than 0.25 mm, which causes the production of inferior cells. Although a gasket having a thickness of the thin part of about 0.25 mm may be used in a relatively small cell such as AA--size, it cannot be used in larger cells such as D--size and C--size, because the gasket cannot be ruptured until the internal gas pressure is built up to 50 kg/cm.sup.2, which is high enough to cause explosion of the cell itself.
Another proposal has been made as shown in U.S. Pat No. 3,314,824 in which a sharp member is provided in the cell adjacent to a gasket to puncture the latter when the internal gas pressure is abnormally increased in the cell. This proposed structure, however, requires the additional sharp member, to thereby increase the production costs of the cell, and also causes a reduction of the effective internal volume of the cell.